In the transmission of digital data, or a bit stream, over a communications channel 2, the bit stream is converted in a transmitter (TX) into an analog signal that is capable of passing through the communications channel. The communications channel may be a radio path, copper wireline or fiber-optic cable. On the basis of the received analog signal, the receiver (RX) performs a recovery of the sent bit stream as error-free as possible. The bit stream reconstruction performed in the receiver is complicated by signal distortion and noise summed with the signal on the communications channel. Due to these side-effects, a portion of the reconstructed bits are erroneous (e.g., on an average, 1 bit per 107 bits may be erroneous).
The signal distortion originating from the transmission path is generally compensated for by means of equalizers that are located in the receiver, the transmitter or partially in both of these. The equalizers may be of a fixed or adaptive type. Respectively, the effect of noise is compensated for by means of different coding techniques such as Reed-Solomon coding, convolution coding, trellis coding, turbo coding and others.
A generally used correction method of channel distortion is the use of a linear adaptive equalizer (FFE). However, a linear equalizer alone may give an insufficient correction on certain channels. This kind of situation may be encountered when the transfer function of the signal band includes zero points, whereby certain frequency components cannot be passed over the communications channel 2. Then, a feedback equalizer is used to compensate for the distortion caused by the spectral nulls of the signal band. Also in a system wherein the channel 2 has no spectral nulls, the use of a feedback equalizer is often advantageous inasmuch it improves the noise tolerance of the system. If the feedback equalizer is located in the receiver, it is called a decision-feedback equalizer (DFE), while an equalizer located in the transmitter is called a Tomlinson-Harashima precoder. A system may also have both a DFE and a TML. Furthermore, the linear equalizer may be situated in the receiver, the transmitter or a portion of the equalizer may be in the transmitter while the other portion is in the receiver.
In the text describing the prior art and the features of the present invention, the following abbreviations are used:
CAPCarrierless amplitude and phase modulationDFEDecision-feedback equalizerFFEFeedforward equalizer, also known as a linear equalizerPAMPulse amplitude modulationQAMQuadrature amplitude modulationRXReceiverTXTransmitterTMLTomlinson-Harashima precoder.
In the following, a digital communications channel is examined in terms of the training phase of its adaptive equalizers. The line code used on the channel may be implemented using either pulse-amplitude modulation (PAM), quadrature-amplitude modulation (QAM) or carrierless amplitude and phase modulation (CAP). In FIG. 1 is shown a model for a system implemented using conventional techniques, wherein the receiver is provided with an adaptive linear equalizer (FFE) and an adaptive decision-feedback equalizer (DFE) (cf. Lee & Messerschmitt). The effect of fixed filters and possible modulation schemes are included in the channel noise model (CHN). The outgoing bit stream is coded into symbols (S) that are sent through the channel 2. In the receiver, the output signal of the channel 2 is processed by equalizers (FFE and DFE), and the decisions on symbols (S′) are made from the equalized signal. The decision resulting in the resolved symbol (S′) is also called the estimated received symbol. Both adaptive equalizers are adapted to the characteristics of the channel 2 during the training period carried out when a connection is being established. The equalizers are also continually adjusted during the period of data transmission in order to compensate for possible changes in the channel 2. The equalizers are adapted and controlled on the basis of the detection error (e) of the receive signal.
In FIG. 2 is shown another system according to the prior art (cf. Lee & Messerschmitt). The receiver has an adaptive linear equalizer (FFE), while the transmitter has a feedback equalizer (of the TML type). During the teaching period, also this system operates in the same fashion as that illustrated in FIG. 1 using a linear equalizer and a decision-feedback equalizer (DFE). At the end of the training period, the tap-weight values of the decision-feedback equalizer (DFE) are transmitted over an upstream auxiliary channel to the transmitter, wherein they are utilized in the configuration of a Tomlinson-Harashima precoder (TML). The linear equalizer (FFE) of the receiver is adjusted during the data transmission state, but due to the fixed configuration of the decision-feedback equalizer (TML) of the receiver, the latter equalizer will not be adjusted.
A benefit of Tomlinson-Harashima precoding over a DFE is that preceding does not cause feedback of a detection error as is the case in a DFE. Particularly when the shape of the amplitude response of the communications channel 2 is such that large values of tap coefficients must be used in the DFE, a really complex problem evokes from the feedback of erroneous decision-making in the detector. In the most serious situations, a single erroneous decision may cause loss of connection when in a system using a DFE.
Generally, changes in the characteristics of a communications channel 2 can be compensated for by adjusting the linear equalizer alone. However, in some cases the communications channel 2 may include analog bandstop filters serving to eliminate radio-frequency interference, for instance. The positions of the spectral nulls caused by the analog bandstop filters in the frequency spectrum may vary as the component values of the filters change with temperature. This kind of variation in the characteristics of the communications channel 2 cannot be compensated for simply by adjusting the linear equalizer. Another complication arises from the incapacity of the system to cope in an optimal manner with varying noise conditions if the decision-feedback equalizer is not adjusted during the data transmission state.
In FIG. 3 is illustrated a prior-art method used for solving the above-described problem. Herein, the system comprises a linear equalizer (FFE), a Tomlinson-Harashima precoder (TML) and decision-feedback equalizer (DFE). During the training period, the system comprises the FFE and the DFE alone. At the end of the training period, the tap coefficient values of the DFE are sent to the precoder (TML) included in the transmitter and the tap coefficient values of the DFE are reset to zero. During the data transmission state, both the FFE and the DFE are adjusted, but not the precoder (TML). A benefit of this arrangement is that the problems associated with such changes in the communications channel characteristics and noise conditions that cannot be coped with merely by adjusting the linear equalizer are overcome, because also the DFE of the receiver can be adjusted during the data transmission state. A disadvantage still remains from the risk of erroneous decision feedback due to the DFE of the receiver. The tap coefficients of the DFE in the receiver may be assumed to have smaller values than in the situation illustrated in FIG. 1 inasmuch a portion of the feedback equalization is performed already in the transmitter. Consequently, also the effect of erroneous decision feedback is less severe than in the configuration shown in FIG. 1. However, the system performance remains substantially dependent on how large changes may occur in the characteristics of the communications channel 2 and system noise condition in regard to the preceding situation prevailed during the training period.
A straightforward approach to improve the system shown in FIG. 2 or 3 would be to compute the incremental values of tap coefficient adjustments in the receiver from the detector error and symbol decisions in the same manner as when adjusting a DFE, but then transmitting the computed incremental values of adjustment over an auxiliary channel of the reverse transmit direction to the transmitter. These incremental adjustment values are then used for updating the tap coefficient values of the precoder in the transmitter. Accordingly, the precoder could be adjusted also during the data transmission state, whereby the receiver DFE would become redundant or the high values of its tap coefficients can be limited. However, it can be shown that this kind of equalizer adjustment method is not practicable in a general case.